Non-volatile magnetic random access memory (MRAM) devices have the potential to replace volatile dynamic random access memory (DRAM) devices and static random access memory (SRAM) devices in some applications. The MRAM devices comprise arrays of cells operating on tunneling magneto-resistance (TMR), colossal magneto-resistance (CMR), and giant magneto-resistance (GMR) technology.
MRAM cells are typically structured around “data” and “reference” layers. The data layer includes a writeable magnetic material, and the reference layer includes a fixed magnetic material. A dielectric layer in between the two has greater or lesser resistance to electrical current depending on whether the magnetic fields from the sandwiching layers are canceling or reinforcing one another.
During a write operation, the magnetization of the data layer can be switched between two opposite states by applying an electromagnetic field through a nearby wire loop. Thus binary information can be stored. The reference layer usually comprises a magnetic material in which the magnetization is pinned. A magnetic field applied to the data layer penetrates the reference layer with insufficient strength to switch the magnetization in the reference layer.
For example, in a TMR cell, the data layer and the reference layer are separated by a thin dielectric layer so that a tunneling junction is formed. The probability that electrons will be able to tunnel through the dielectric layer depends on the direction of the magnetization in the data layer relative to the direction of the magnetization in the reference layer. Therefore, the structure is “magneto-resistant” and information can be stored and retrieved by reading the magnitude of tunneling currents thereafter able to pass through the memory cell.
In general, it is of advantage that the magnetic memory cells are as small as possible to increase memory density and reduce cost. However, as cells become smaller, thermal stability issues become more important. To ensure that stored information is not lost because of random switching induced by environmental influences, it is necessary that the data layers of small magnetic memory cells are such that the magnetic field strength that is required for switching the magnetization is higher than that for larger memory cells. Unfortunately the necessity to generate the larger fields strength makes switching of the memory cells during the write operation more difficult.
It is known that increasing the temperature of the magnetic memory cell lowers the magnetic field strength that is required for switching. Further, when an electrical current passes through the magnetic memory cell, heat is developed in the cell. However, the developed heat is easily conducted through the bit lines away from the memory cell and therefore cannot be utilized to facilitate switching of the magnetic memory cell.
There is therefore a need for a magnetic memory device in which loss of heat from the magnetic memory cell is reduced and therefore the heat can be utilized to facilitate switching.